Tuning means for slot radiator



United States Patent TUNING MEANS FOR SLOT RADIATOR Matti S. O. Siukola,Woodlynne, N.J., assignor to Radio Corporation of America, a corporationof Delaware Filed Oct. 16, 1957, Ser. No. 690,471

2 Claims. (Cl. 343-467) The invention relates to a tuning device for aradiating slot in a metallic surface, and has for an object to providean improved tuning device that permits the resonant frequency of theslot to be easily varied.

The tuning device is primarily intended to be used with -a radiatingslot or slots in a transmission line or antenna. However the device canbe used to vary the resonant frequency of any slot. Generally, each ofthe slots is relatively narrow and has sides that are substantiallylonger than its width as measured across the respective ends. The tuningdevice for each slot comprises at least one conductive adjustable platethat is positioned near one of the ends of the slot. A portion of theplate is arranged so that it projects over a portion of the slot nearthe one end thereof, thus eifectively changing the electricalcharacteristics of the slot mainly by restricting the magnetic field. Ifdesired, a second similar adjustable plate may be positioned near theother end of the slot. Since the plates are adjustable, the electricalcharacteristics, and hence the resonant frequency, of the slot can beeasily and quickly varied.

The invention is explained in detail in connection with the accompanyingdrawing, in which:

Figure 1 shows an elevational view of a slot antenna having radiatingslots with which the tuning devices of the invention may be used;

Figure 2 shows an enlarged plan view of one of the slots shown in Figure1, and also shows one embodiment of the adjustable plates of the tuningdevice;

Figure 3 shows an enlarged plan view of a slot having another embodimentof the adjustable plates of the tuning device, and also shows aprotective cover for the slot; and

Figure 4 shows a cross-sectional view taken along the lines 4-4 ofFigure 3.

Referring to the drawing, Figure 1 shows a traveling wave antenna whichincludes a hollow, cylindrical outer conductor 12, and a cylindricalinner conductor 14 concentrically positioned within the outer conductor12. The traveling wave antenna 10 is designed to radiate radio frequencysignals in the television band, and is preferably mounted in a verticalposition on some hollow structure such as a building or a tower. InFigure 1, the antenna 10 is shown mounted in or supported by a suitablepedestal 20 which may house or contain suitable means for applying radiofrequency energy to the antenna 10. The outer conductor 12 of theantenna 10 extends above the pedestal 20, and is provided with aplurality of elongated slots 16, each positioned with its long dimension(about one-third wavelength at the operating frequency) substantiallyparallel to the longitudinal axis of the antenna 10. The slots 16 aredesigned to have a relatively high Q, the exact value depending upon thegain and bandwidth to be provided :by the antenna 10. It has been foundthat a slot having an enlarged width at the ends provides a higher Qthan a slot having a uniform width throughout. The configuration of sucha slot can be seen in Figure l, and is similar to a dumbell. The slots16 are arranged in a spiral about the outer conductor 12 of thetraveling wave antenna 10. There is one pair of slots 16 positioned atthe same horizontal level every quarter wavelength at the operatingfrequency up the vertical height of the antenna 10, the slots 16 at eachlevel being diametrically opposite each other. This has not been shownin detail in Figure 1 in order not to detract from the clarity of theillustration. Each successive or higher pair of slots 16 lies in avertical diametrical plane that forms an angle of substantially degreeswith the plane in which the adjacent lower pair of slots 16 lies.

Radio frequency energy may be applied to the antenna 10 by any suitablemeans, such as a coaxial transmission line whose outer conductor isconnected to the outer conductor 12 of the antenna, and whose innerconductor is connected to the inner conductor 14, of the antenna 10. Itis preferred that this radio frequency energy, when excited in theantenna 10, be in the TEM mode. This energy is applied at the base orpedestal 20 of the antenna 10 and travels upward through the antenna 10toward the top. Equal amounts of this radio frequency energy are coupledto the slots 16 by any suitable means. In order that all of the energyin the antenna 10 be radiated, terminating slots 18 are provided at thetop of the antenna 10. The terminating slots 18 are positioned in thesame manner as the radiating slots 16, but have a substantiallyrectangular configuration and uniform width throughout so as to providea relatively low Q slot. A complete termination for the antenna 10 isprovided by some suitable means, such as a plate 22, which shortcircuitsthe inner and outer conductors 14, 12 at the proper point at the top ofthe antenna 10.

Antennas such as shown in Figure 1 have been built in about 30 footsections from lengths of 60 feet to feet overall with a diameter ofapproximately 10 to 20 inches. Because of such large physicaldimensions, the mechanical tolerances must be quite lenient. Thus incutting the slots 16, 18 in the outer conductor 12 of the antenna 10,certain tolerances must be permitted. These tolerances, while necessary,result in slots which are not all exactly resonant to the desiredfrequency of operation. Thus some means for tuning the slots must beprovided so that they resonate at the desired operating frequency. Suchan arrangement is shown in Figure 2.

Figure 2 shows an enlarged plan view of one of the slots 16 in the outerconductor 12 of the antenna 10. The slot 16 has enlarged portions 24 atits ends, these enlarged portions 24 serving to provide the slot 16 witha higher Q than a conventional rectangularly shaped slot. As mentioned,the necessary manufacturing tolerances usually result in the slots 16having different resonant frequencies, the differences being appreciablein some instances where large antennas are constructed. In order thatthe slot 16 may be tuned to the desired resonant frequency,rectangularly shaped, conductive plates 26 are provided at the ends ofthe slot 16. These plates 26 are attached or fastened to the outerconductor 12 near the enlarged portions 24 of the slot 16 so that aportion of each of the plates 26 projects over or covers the enlargedportions 24 of the slot 16. The plates 26 may be fastened by anysuitable means, such as bolts which pass through elongated slots in theplates 26. However, it is not necessary that the plates 26 be directlyconnected to the outer conductor 12. As long as the plates 26 be in aposition to vary the magnetic field of the slot 16, they may be mountedin any convenient fashion. This is one advantage provided by theinvention. These elongated slots in the plates 26 permit the plates 26to be adjusted back and forth until the slot 16 has the desired resonantfrequency. A change in the position of the plates 26 also changes themagnetic field of the slot 16, thus changing the resonant frequency ofthe slot 16. Although the plates 26 are shown as being rectangular inshape, other shapes can be used as well. For example, the long sides ofthe plates 26 may be tapered. The terminating slots 18 in Figure 1 mayalso have such tuning plates.

While the tuning plates 26 perform satisfactorily to change the resonantfrequency of the slot 16, the problem of making the slot 16 weatherproofis also present. An arrangement for providing tuning plates 26 as wellas for making the slots 16 weatherproof is shown in the plan viewinFigure 3. A longitudinal cross-sectional view, taken along the lines44 in Figure 3, is shown in Figure 4. A conductive bracket 30 isfastened to the outer conductor 12 near each of the enlarged portions24. The brackets 30 include a tuning plate 26' and a retaining element28 which is positioned above, but spaced from the tuning plate 26. Thespace between the plate 26 and the retaining element 28 serves toreceive and hold a suitable covering 32 for the slot 16. In effect, thisspace provides a groove positioned with respect to the slot 16 to holdthe member 32 in slot covering position. The covering 32 for the slot 16may be made of any suitable dielectric material, such as polyethylene orpolystyrene. The brackets 30 may be attached to the outer conductor 12by suitable means such as bolts which pass through elongated slots in arear projection 34. The bracket 30 is mounted at each end of slot 16 soas to hold the covering 32. After the bracket 30 is adjusted so that thetuning plate 26' provides the desired resonant frequency for the slot16, it is then fastened into position so that it holds the covering 32tightly at its ends. The bracket 30 provides an improved way of holdingthe covering 32, as well as providing tuning means for the slot 16,because it projects above the surface of the outer conductor 12 aminimum amount. Such a feature is highly desirable in reducing the windloadings required for the antenna structure.

In addition to using the plates for changing the resonant frequency ofthe slots, a magnetic material, such as a low loss powdered iron core,may be placed in the slots to change their resonant frequency. Theplates may be considered to restrict the magnetic field of the slots,while the magnetic material may be considered to increase the magneticfield of the slots, both changes serving to change the resonantfrequency of the slots.

The tuning device described can be produced at relatively low cost,since it can be made of thin metallic plates. Further, these platesprotrude only slightly above the surface of the antenna, thusmaintaining wind loads at a minimum. In addition, the plates present noelectrical contact problems such as presented with capacitor plates.

4 And finally, insulation problems are reduced because the plates are atthe ends of the slot whichis a low voltage point.

The invention claimed is:

l. A conductive member presenting an exposed surface and having a slottherein terminating in said exposed surface for radiating radiofrequency energy, said slot having sides that are substantially longerthan its ends and being enlarged at said ends to form a substantiallydumbbell-shaped slot so as to transfer radio frequency energytherethrough, means for changing the resonant frequency of said slot byvarying the magnetic field of said slot, said last named meanscomprising a pair of conductive members each positioned with respect tosaid slot to project longitudinally along said slot from an end thereof,each of said conductive members being narrower than the width of saidslot whereby to cover a longitudinally extending portion of said slot,each of said conductive members being provided with a groove extendinglaterally with respect to said slot, and a dielectric covering memberbeing positioned in said grooves to cover said slot.

2. A conductive member presenting an exposed surface and having a slottherein terminating in said exposed surface for radiating radiofrequency energy, saidslot having sides that are substantially longerthan its ends and being enlarged at said ends to form a substantiallydumbbell-shaped slot so as to transfer radiofrequency energytherethrough, means for changing the resonant frequency of said slot byvarying the magnetic field of said slot, said last named meanscomprising a pair of conductive members each positioned with respect tosaid slot to project longitudinally along said slot from an end thereof,each of said conductive membersbeing narrower than the widthof said slotwhereby to cover a longi' tudinally extending portionof said slot, eachof said conductive members being provided with a retainingrelement whichis positioned above, but spaced from said conductive members and adielectric covering member extending over said slot and being positionedbetween said spaced retaining elements and said conductive members.

References Cited in the file of this patent UNITED STATES PATENTS2,507,528 Kandoian May 16, 1950 2,512,468 Percival June 20, 19502,629,051 Lindenblad Feb. 17, 1953 2,658,143 Fiet et al. Nov. 3, 19532,763,783 Lorcnzen Sept. .18, 1956 FOREIGN PATENTS 600,433 Great BritainApr. 8, 1948

